The electromagnetic spectrum has frequencies associated with it in which electromagnetic radiation may be radiated. The electromagnetic spectrum can be divided into frequency regions that exhibit common properties useful in science and technology. For example, the audible range of frequencies is approximately between 20 Hertz (Hz) to 20,000 Hz which humans can here. There is a radio frequency band of the electromagnetic spectrum which is allocated for radio, including cellular phones, and television communication systems. Electromagnetic radiation in the radio frequency bands tends to bend around, reflect off of and pass through objects, and thus, is favorable to communication systems. There is a narrow band referred to as the visible spectrum between 3.95×1014 Hz to 7.90×1014 Hz over which the radiant energy is visible to a human eye. The visible spectrum may divided into frequencies of color. Just below the visible spectrum is the infrared (IR) frequency spectrum which is in the range between 3×1011 Hz to 4×1014 Hz which is not visible to the human eye. More typically, the frequency of electromagnetic radiation in the IR frequency spectrum is expressed in wavelengths because of its light properties. In this case, the wavelength of light (λ) is proportional to the inverse of the frequency (f) and can be expressed in equation form as λ=C/f where C is the speed of light.
Infrared (IR) radiation having properties of light travels in a straight, or line-of-sight, path. IR radiation is blocked by opaque objects and typically reflects well off of only hard, mirror-like surfaces. Thus, electromagnetic radiation in the IR frequency spectrum, referred to as IR radiation, is not typically used in communication systems.
A communication system can be full duplex or simplex. A full duplex communication system provides constant bi-directional communication between users such as a telephone. A simplex communication system provides bi-directional communication between users but not at the same time. A user at each end selects whether he or she wants to talk or listen. A user at one end can not both listen and talk at the same time in a simplex communication system.